RESEARCH ARTICLE
OPTICS
Hybrid bilayer plasmonic metasurface efficiently
manipulates visible light
2016 © The Authors, some rights reserved;
exclusive licensee American Association for
the Advancement of Science. Distributed
under a Creative Commons Attribution
NonCommercial License 4.0 (CC BY-NC).
10.1126/sciadv.1501168
Fei Qin,1* Lu Ding,2* Lei Zhang,1* Francesco Monticone,3* Chan Choy Chum,2 Jie Deng,2 Shengtao Mei,1,4
Ying Li,5 Jinghua Teng,2 Minghui Hong,1 Shuang Zhang,6 Andrea Alù,3 Cheng-Wei Qiu1,5†
INTRODUCTION
The ability of bending and molding propagating light beams is of
great importance in science and technology. Conventionally, light deflection relies on the gradual phase accumulation along the light path
by using, for example, lenses and prisms. However, the limited range of
permittivity and permeability values of natural materials imposes significant limitations in the design of these conventional optical components,
for example, in terms of their thickness, which is generally comparable
to the operating wavelength, hence hindering their integration with
nanophotonic circuits. Metamaterials, composed of artificial nanostructures, have been shown to allow extreme control of electromagnetic
fields, much beyond what is attainable with naturally occurring materials. In particular, metamaterials can be tailored to exhibit exotic and
intriguing effects, such as negative refraction, perfect lensing, and
cloaking (1–3). However, the fabrication of three-dimensional (3D)
large-scale metamaterials remains a big technological challenge. Another
major problem in plasmonic metamaterials is represented by the strong
energy dissipation in metals, especially at visible frequencies.
Recently, planarized 2D metamaterials, or “metasurfaces,” have
gained increasing attention, because they may allow realization of
anomalous effects, while relaxing the limitations and challenges of
bulk metamaterials (4–6). Notably, ultrathin metasurfaces have been
suitably designed to deflect an impinging beam into anomalous refraction channels, obeying “generalized Snell’s laws,” by imparting a
controlled gradient of phase discontinuities along the metasurface
1
Department of Electrical and Computer Engineering, National University of Singapore,
117583 Singapore, Singapore. 2Institute of Materials Research and Engineering, Agency
for Science, Technology and Research, 117602 Singapore, Singapore. 3Department of
Electrical and Computer Engineering, The University of Texas at Austin, Austin, TX 78712,
USA. 4Graduate School for Integrative Sciences and Engineering, National University of
Singapore, 117456 Singapore, Singapore. 5SZU-NUS Collaborative Innovation Center for
Optoelectronic Science and Technology, Shenzhen University, Guangdong 518060, China.
6
School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK.
*These authors contributed equally to this work.
†Corresponding author. E-mail: [email protected]
Qin et al. Sci. Adv. 2016;2:e1501168
1 January 2016
(7–12). The thickness of these structures is far smaller than the operational wavelength, allowing, in principle, the miniaturization and
integration of several optical components and systems. Many different
metasurfaces, including plasmonic and dielectric designs, as well as
ultrathin and finite-thickness configurations, have been reported
(13–26). Metasurfaces operating in reflection mode have been shown
to exhibit relatively higher efficiencies. However, reflective metasurfaces involve more complicated optical setups and may introduce certain inconvenience in many applications. Particular interest has been
devoted to ultrathin metasurfaces working in transmission mode,
based on arrays of metallic nanoantennas, or Babinet-inverted designs,
namely, arrays of apertures in a metallic sheet (4, 5, 9, 11, 14, 15).
However, Monticone et al. (13) theoretically demonstrated in their
study (and Supplementary Materials therein) that any passive metasurface with infinitesimal thickness can fully manipulate the phase of
cross-polarized transmitted light at only 25% of the total impinging
energy. Realistic implementations have shown even lower efficiency
in the order of a few percent (13–15). Although high diffraction efficiency (~80%) was reported using Si metasurfaces (23), it follows the
similar definition of extinction ratio, which describes the ratio of the
power of anomalous components over the transmitted power. It is worth
noting that it is still challenging to achieve high conversion efficiency
of the overall system, corresponding to the power of converted anomalous components versus the total incident power, that is, transmission coefficient multiplied with diffraction efficiency. In the study
of Lin et al. (23), on the basis of the presented data, the best two cases
(at 550 and 650 nm) yield ~20% in conversion efficiency because the
highest transmission coefficient and highest diffraction efficiency arise
at the different wavelengths.
It has been reported that, by using isotropic metamaterial Huygens
surfaces, consisting of cascaded metasurface layers, it is possible to
achieve high efficiency in anomalous deflection of the incident light
at telecommunication wavelength around 1.5 mm, thanks to the interference of the fields radiated by effective electric and magnetic surface
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Metasurfaces operating in the cross-polarization scheme have shown an interesting degree of control over the
wavefront of transmitted light. Nevertheless, their inherently low efficiency in visible light raises certain concerns for
practical applications. Without sacrificing the ultrathin flat design, we propose a bilayer plasmonic metasurface
operating at visible frequencies, obtained by coupling a nanoantenna-based metasurface with its complementary
Babinet-inverted copy. By breaking the radiation symmetry because of the finite, yet small, thickness of the proposed
structure and benefitting from properly tailored intra- and interlayer couplings, such coupled bilayer metasurface
experimentally yields a conversion efficiency of 17%, significantly larger than that of earlier single-layer designs,
as well as an extinction ratio larger than 0 dB, meaning that anomalous refraction dominates the transmission
response. Our finding shows that metallic metasurface can counterintuitively manipulate the visible light as efficiently as dielectric metasurface (~20% in conversion efficiency in Lin et al.’s study), although the metal’s ohmic loss
is much higher than dielectrics. Our hybrid bilayer design, still being ultrathin (~l/6), is found to obey generalized
Snell’s law even in the presence of strong couplings. It is capable of efficiently manipulating visible light over a
broad bandwidth and can be realized with a facile one-step nanofabrication process.
RESEARCH ARTICLE
rator. Lift-off process is avoided (further details are discussed in Materials and Methods). The fabrication process of our bilayer metasurface
is arguably easier than the fabrication of single-layer metasurfaces composed of nanoantenna arrays, which requires to carefully lift off the
metal on the unexposed photoresist. The fabrication of the bilayer
metasurface with such one-step process is therefore more viable and
cost-effective, largely beneficial for practical applications. A top-view
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currents (7, 10, 27). These designs generally require sophisticated fabrication processes and unit cells, which result in great challenges to
apply this conception in the visible range. These works primarily
focused on the manipulation of the copolarized component of the
transmitted field, and it is imperative to find an accessible recipe to
achieve a highly efficient manner to manipulate cross-polarization directly in visible regime using ultrathin metasurfaces. The resultant recipe should be easily and readily scaled into longer wavelengths.
Here, we propose and experimentally demonstrate an efficient bilayer plasmonic metasurface working in transmission mode at visible
frequencies. Owing to the properly tailored intra- and interlayer couplings, significantly larger cross-polarization conversion efficiency of
up to 36.5% has been achieved in simulations and 17% in experiments.
Notice that the 25% theoretical limit for cross-polarized conversion
efficiency does not apply to our metasurface design because of its
finite, yet small, thickness, which allows breaking the radiation symmetries of ultrathin metasurfaces. The results reported in this paper
represent a significant improvement in the manipulation of visible
light by metasurfaces in terms of conversion efficiency and extinction
ratio. The structure proposed here has a total thickness of only 130 nm
(~l/6) and has been realized by using a facile one-step fabrication
process, as described in the following.
RESULTS AND DISCUSSION
The basic structure proposed here consists of two physically separated
plasmonic layers: the top layer is a 2D array of gold V-shaped nanoantennas, whereas the bottom layer is a 2D array of V-shaped Babinetinverted apertures in a gold film, as shown in the inset of Fig. 1A. The
two layers are separated by conformal pillars made of hydrogen silsesquioxane (HSQ) negative E-beam resist. Similar complementary configurations with different shapes have been investigated previously for
different applications, such as biosensing (28–30), high extinction ratio
polarizers (31–33), high-resolution printing of color images (34, 35),
and dark plasmonic resonance excitation (36). To design a metasurface for cross-polarization conversion and beam deflection, we define
a supercell consisting of six basic elements, which provide similar
cross-polarization transmission amplitude and incremental transmission phase with p/3 steps. Subunits 1 to 3 are constructed by properly
changing the length and the angle between the two arms of nanoantennas, similar to the design in studies of Yu et al. (4) and Ni et al.
(5) (the symmetry axis of the V-shaped antennas is along 45° relative to
the x axis). Subunits 4 to 6, instead, are constructed by simply rotating
the symmetry axis of subunits 1 to 3 90° clockwise. This provides a
full coverage of the transmission phase range. The subunit cell period
is 150 nm; therefore, the overall dimension of the supercell is 900 ×
150 nm. The height H of the HSQ pillars and the thickness T of the
gold layers (inset of Fig. 1A) are 100 and 30 nm, respectively. The
separation between the two plasmonic layers D is therefore 70 nm,
much smaller than the working wavelength in the visible range, implying a strong coupling between top and bottom metasurfaces, similar
to gap-surface plasmon effects discussed by Pors et al. (12). The total
thickness of the proposed device is only 130 nm (~l/6).
A key feature of our bilayer metasurface is its ease of fabrication, as
shown in Fig. 1B (i to iii). By using electron beam lithography (EBL),
we patterned the structure on a SiO2 substrate with a layer of HSQ
100 nm thick, followed by a gold deposition with an E-beam evapoQin et al. Sci. Adv. 2016;2:e1501168
1 January 2016
Fig. 1. High-efficiency beam bending of visible light with a complementary bilayer metasurface. (A) Working principle of the designed structure. The light spots on the screen in the foreground are the combined
real photo images of anomalous light deflection for different wavelengths.
Inset: Sketch of the subunit cell, which consists of one layer of gold nanoantennas on the top and its Babinet-inverted pattern at the bottom,
separated by conformal HSQ pillars. L and W are the length and width of
the nanoantenna arms, respectively; q is the angle between two arms; T
is the thickness of the gold film; H is the height of the HSQ pillar; and D
is the nominal space between the top and bottom layers. (B) Schematic
representation of the fabrication procedure. (i) A 100-nm-thick layer of
HSQ is spin-coated onto a SiO2 substrate. (ii) Patterning using EBL. (iii) A
30-nm-gold film is deposited on the sample by electron beam evaporator. (iv) Top view of SEM image of the bilayer metasurface. The supercell
(yellow) comprises six V-shaped nanoantennas. The supercell repeats with
a periodicity of 900 nm along x axis and 150 nm along y axis. SU, subunit.
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than (in simulation) the dielectric counterpart, which will be elaborated with details in Conclusions.
The measured cross-polarization conversion efficiency against
wavelength and angle is plotted in Fig. 2C. The relationship between wavelength and refraction angle is in good agreement with
the theoretical predictions calculated by the generalized Snell’s law
l df
(4), as the dash-dotted line shown in Fig.
nt sin qt − ni sin qi ¼
2p dx
1A. To better visualize the deflection effect, we also projected the
transmitted light onto a sheet of white paper behind the sample in
our experiments. As seen in Figs. 1A and 2C, there are two light spots
on the screen at each wavelength, corresponding to the normal and
anomalous refracted beams. From these optical images, we can clearly
see that the relative intensity of the anomalous spots grows with
increasing wavelength. When the wavelength of the linearly polarized
light reaches 700 nm, the anomalous refracted spot is even brighter
than the normal one, which can be appreciated by the naked eye as
shown in Fig. 1A.
In all previous realizations of single-layer metasurfaces based on
nanoantenna arrays, it was generally assumed that the coupling between
adjacent unit cells was negligible at the design stage (4, 8, 11). Instead,
for our bilayer metasurface, the intralayer coupling effect not only cannot
Fig. 2. Performance of the bilayer plasmonic metasurface. (A) Experimental results of the conversion efficiency and extinction ratio. (B) Simulated cross-polarized Ex component of the transmitted field of a bilayer
metasurface at 770 nm, which is the peak value position of the conversion
efficiency in simulation. The geometrical parameters used in the simulations are obtained by measuring the fabricated sample with SEM and
AFM techniques. The lengths of each antenna and the angles between
the two arms for unit cells 1 to 3 are as follows: Li = 130, 125, and 120 nm,
and qi = 70°, 90°, and 110°, respectively; width of the nanoantennas W =
55 nm, thickness of the gold film T = 30 nm, and height of the HSQ pillar
H = 100 nm. Unit cells 4 to 6 are constructed by rotating the symmetry
axis of unit cells 1 to 3 90° clockwise. A sketch of the supercell consisting of
six subunits is shown at the bottom of the field pattern, where the actual
sample would be placed. (C) Relationship of transmittance of anomalous
light on the wavelength and the anomalous refraction angle. False-color
map indicates the experimentally measured intensity of the anomalous refracted beam. Inset: Optical image of the transmission spots at a wavelength of 720 nm.
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scanning electron microscopy (SEM) image of our bilayer metasurface
sample with Gx = 900 nm and Gy = 150 nm is shown in Fig. 1B (iv),
where Gx and Gy are the periodicities of the supercell along the x and y
axes, respectively. A 3D surface profile measured by an atomic force
microscope (AFM) showed that the thickness of the fabricated structure is around 100 nm, as shown in fig. S1.
Two important figures of merit for any spatial modulators are the
peak conversion efficiency and the extinction ratio. Here, the conversion efficiency is defined by normalizing the transmitted power of the
anomalous refracted beam to the power transmitted through a bare
SiO2 substrate. The extinction ratio, in turn, is expressed as ER =
10 log(PEx/PEy)dB, where PEx and PEy indicate the transmitted power
that undergoes anomalous refraction with cross-polarization and
normal refraction with copolarization, respectively. Therefore, extinction ratio larger than 0 dB means that the anomalous transmission
dominates. Figure 2A shows the measured figures of merit of the fabricated bilayer metasurface. Despite the discretization and other imperfections, experimental data show that the conversion efficiency
from Ey to Ex components reach a peak value of 17%, which is significantly larger than that of previous experiments based on single-layer
metasurfaces (4, 11). It inevitably has some deviation between experiments and simulations because of the fabrication and characterization challenges (for further details, see figs. S2 and S3). Despite that,
the experimental extinction ratio still exceeds 0 dB in the wavelength
range from 680 to 720 nm, with a peak value of 0.73 dB at 700 nm,
which is much larger than any previous transmitting metasurface design to the best of our knowledge, demonstrating the potential of the
proposed bilayer metasurface.
With the geometrical parameters directly measured from the fabricated samples, as shown in Fig. 2B, the transmitted electric field
distribution for the cross-polarized component Ex was calculated at
770 nm, corresponding to the peak value position of the conversion
efficiency in simulation, which clearly demonstrates an anomalously
deflected planar wavefront. Because the period of the supercell in
the x direction is larger than the working wavelength, multiple diffraction orders may be allowed to propagate. However, if the metasurface
is properly designed, only the diffraction order responsible for the
anomalous refraction will be efficiently excited, and other residual diffraction orders will not seriously affect the conversion efficiency or the
planarity of the wavefront (see fig. S4).
Recently, dielectric metasurfaces with high efficiency have been
demonstrated (23, 37, 38). Notice that the diffraction efficiency for circularly polarized light at 75% in a visible region, reported by Lin et al.
(23), is defined as the ratio of the power converted into anomalous
component over the power of the overall transmittance, similar to
the extinction ratio used in our work. Apart from the extinction ratio,
the conversion efficiency would be another important consideration,
which marks the capability of converting total incident power into the
transmitted anomalous component. Plasmonic metasurfaces with high
conversion efficiency and positive extinction ratio have not been reported for visible light. However, the poly-Si metasurface, when thin
enough as used in the study of Lin et al. (23), can be decently transparent and achieve quite high conversion efficiency in visible light.
This may further consolidate the widely accepted perception that dielectric metasurfaces outperform the plasmonic ones thanks to the
lower ohmic loss. Counterintuitively, our work demonstrated that
our coupled bilayer plasmonic metasurface, still being ultrathin, could
behave almost equivalently to (in experiment) or even slightly better
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surface layers. For a single-layer ultrathin nanoantenna metasurface,
only transverse electrical currents Js ¼ n H a are induced (Ha is
the magnetic field on the surface, and n indicates the unit vector
normal to the metallic screen, pointing toward the half-space of interest) and are bound to symmetrically radiate on both sides of the metasurface, fundamentally limiting the ratio of transmitted energy. In
this sense, a single ultrathin metasurface is equivalent to a shunt
(parallel) reactance, which can only introduce a discontinuity on the
transverse magnetic field, whereas the transverse electric field remains continuous.
As a result, a single ultrathin metasurface supporting purely electric
currents cannot fully manipulate the transmitted waves. To determine
a discontinuity of the transverse electric field, it is necessary to introduce an impedance element in series, which necessarily implies a
structure with finite thickness. A metasurface structure that includes
both parallel and series impedance elements (for example, in the form
of a P network, or T network) can therefore determine an arbitrary
discontinuity in the propagating electromagnetic field, hence allowing full control of the transmission coefficient. In the bilayer structure proposed in this paper, the nanoantenna metasurface and the
nanoaperture metasurface represent shunt impedances of capacitive
and inductive type, respectively (if the nanoantennas and nanoapertures are smaller than their resonance length). In addition, the metasurfaces are separated by a small gap that introduces an equivalent
series capacitance. Thanks to this combination of parallel and series
elements, the proposed metasurface structure is therefore able to manipulate the transmitted beam to a large extent, breaking the 25%
efficiency barrier of single ultrathin metasurfaces. It should also be
stressed that, although dual-layer metasurfaces exhibiting strong
chiral effects and efficient polarization conversion have been reported before (39–42), our goal is not only to achieve high conversion efficiency
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be overlooked but also plays an important role in the spatial modulation
process. In our structure, the subunits are arranged close to each other,
leading to a well-confined electric field at the gap between adjacent
nanoantennas (see fig. S5). As the subperiodicity L decreases from
180 to 150 nm, the conversion efficiency markedly increases from 9
to 29% (Fig. 3A), although the filling fraction (structural area versus
the total surface area) only increases from 21 to 30.3%. It indicates that
the intralayer coupling plays an important role in enhancing the manipulation performance of the proposed metasurface, a significant
difference with respect to earlier metasurface designs, which did not exhibit such sensitivity to the distance between neighboring elements. This
feature is evident in Fig. 3 (B and C), which considers only one of the
two layers at a time. Further decreasing the subperiodicity will indeed
support even higher conversion efficiency. However, it would also make
the fabrication process very difficult; therefore, subperiodicity is chosen
to be 150 nm in the experiment. Conversely, the extinction ratio of all
three types of metasurfaces is almost insensitive to the variation in subperiodicities, as seen in Fig. 3 (D to F). However, the extinction ratio of
the bilayer metasurface can reach a peak value of 12 dB and is larger
than 0 dB over a broad wavelength range (see Fig. 3D), which is favorable for practical applications. In contrast, either single-layer nanoantenna metasurface or the Babinet-inverted type metasurface only exhibits
peak value around 0 dB.
As discussed above, the conversion efficiency of the proposed
bilayer metasurface can reach a value of 29%, which even exceeds
the theoretical limit for single-layer metasurfaces (13). This is possible because the designed metasurface interacts with the impinging
light over a finite thickness, breaking the radiation symmetries that
fundamentally limit the performance of infinitesimally thin structures
(13). In particular, for the structure considered here, the radiation symmetry is broken because of the structural asymmetry of the two meta-
Fig. 3. Comparison of three different types of metasurfaces, showing different sensitivities to the subperiodicity (that is, the distance
between neighboring cells). For ease of comparison, in these simulations, the same parameters were used for all the three types of metasurfaces.
(A to C) Dependence of the conversion efficiency on the subperiodicities for three types of metasurfaces as the subperiodicities increase from 150 AQ17
to 180 nm. (D to F) Dependence of the extinction ratio on the subperiodicities for three types of metasurfaces as the subperiodicities increase from
150 to 180 nm.
Qin et al. Sci. Adv. 2016;2:e1501168
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but also to locally control the phase of the cross-polarized transmission over the entire 2p phase range, which is challenging with conventional design principles. In particular, the complementary nature
of the two metasurfaces, one capacitive and the other inductive, is beneficial to achieve large coverage of the transmission phase, consistent
with the results of Monticone et al. (13). In this context, our results
clearly demonstrate that “two highly coupled plasmonic metasurfaces
can do much better than one,” in analogy with the recent investigation
of bilayer 2D materials (43).
Fig. 5. Field and current distributions for one selected subunit (that is, SU2 in Fig. 1B, iv) under antisymmetric excitation. (A) z components
of electric and magnetic field distributions on the top and bottom layers of the bilayer metasurface, respectively. The displacement current density J
at resonance on both arms of the HSQ pillar is represented by vector arrows, where green and orange indicate whether the z component of the
AQ18AQ19 displacement current (JZ) is toward −z and +z directions, respectively. (B and C) Transverse electrical current (Js) and equivalent transverse magnetic
current (Jm) of the bilayer metasurface. The position of the V-shaped nanoantenna on the top and the Babinet-inverted aperture on the bottom are
indicated by the white outlines. The amplitudes are represented by the color of the arrows with the same relative scale for Js and Jm, respectively.
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Fig. 4. Simulated transmittance with the optimized structure. The parameters were defined as follows: For the length of each antenna and the
angle between the two arms for units 1 to 3, Li = 140, 130, and 125 nm,
and qi = 70°, 90°, and 110°, respectively; W = 50 nm, T = 30 nm, and H =
100 nm. Unit cells 4 to 6 are constructed by rotating the symmetry axis of
unit cells 1 to 3 90° clockwise.
The geometrical parameters used in the simulations of Fig. 3 are
extracted from the fabricated sample by using AFM and SEM techniques. The conversion efficiency may be further increased to 36.5%
with a proper optimization of the structure, as shown in Fig. 4. We
also performed numerical simulations to investigate the near-field distributions of our metasurface structure and its resonance properties.
In Fig. 5A, the z components of electric and magnetic fields and the
displacement current density are plotted for the second subunit of the
designed supercell at wavelength 815 nm under antisymmetric excitation (the polarization of incident light is perpendicular to the
symmetry axis of the V-shaped antenna). A unit cell in a uniform periodic structure has been considered here (simulated by imposing periodical boundary conditions on the sides of the cell). According to the
Babinet principle, if the direct structure has an E-field resonance mode,
the complementary structure will exhibit a corresponding H-field
resonance mode at the same frequency (44–46), assuming that the
structures are made of infinitely thin perfectly conducting sheets, and
the wave propagates perpendicularly toward the sheets. Although real
metals can no longer be considered perfectly conducting at optical frequencies, the field distributions of the V-shaped nanoantennas on the
top and their complementary on the bottom are indeed in agreement
with the Babinet principle, as seen in Fig. 5A. The E-field distribution
on the top layer shows an antisymmetric resonance, whereas the H-field
shows a symmetric resonance at the bottom Babinet structure. We also
show the displacement current density at the resonant frequency on both
arms of the HSQ pillar, where the green (orange) arrows indicate that Jz is
toward the −z (+z) direction. The resonant displacement current
determines strong interlayer coupling, which contributes to the high efficiency exhibited by the proposed metasurface design.
According to the surface equivalence theorem (47), small apertures in a metallic screen, as in the case of Babinet-inverted metasurfaces, can be treated as arrays of magnetic current elements. In
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wavelength can be adjusted by changing the width and length of the
antennas (see figs. S7 and S8). This mechanism may be extended to even
higher frequencies by using silver or aluminum bilayer metasurfaces.
CONCLUSIONS
In summary, we numerically and experimentally demonstrated a
bilayer metasurface design composed of a nanoantenna-based metasurface paired with its Babinet-inverted counterpart, connected via
conformal HSQ pillars. Thanks to the combination of parallel and series impedance elements (namely, the two metasurfaces, one capacitive, the other inductive, and the gap between them), which allows for
suitable breaking of the radiation symmetries of the structure, our
hybrid design can achieve high conversion efficiency and high extinction
ratio simultaneously, with a 36.5% conversion efficiency achieved in
simulation and 17% in experiment. It is noted that the dielectric metasurface (23) operating in visible light provides about 20% conversion
efficiency (transmission multiplied by diffraction efficiency) in both
simulation and experiment, on the basis of the best two cases captured
at 550- and 650-nm wavelengths in Fig. 3D and fig. S5C of the study
by Lin et al. (23). At the same time, our extinction ratio can be as high
as 0.73 dB experimentally, also significantly larger than that of previous works. Although there are strong mutual couplings (in contrast to
those previously reported metasurface without mutual coupling) between neighboring elements, the generalized Snell’s law is still applicable. Our metasurface design can work over a broadband range through
visible and near-infrared frequencies. The overall thickness of the bilayer structure is only 130 nm (l/6), and its fabrication process is significantly more viable. It is expected that the overall efficiency of our
hybrid plasmonic metasurface can be markedly improved further with
better optimization and more precise nanofabrication, which potentially exceed the performance of all previously demonstrated metasurfaces in the transmission at visible range.
MATERIALS AND METHODS
Sample preparation
The bilayer metasurface was fabricated using EBL, followed by the deposition of one layer of gold by E-beam evaporator, as shown in Fig. 1B.
The structure was fabricated on a 1-mm-thick quartz substrate. After
thoroughly cleaning the substrate by acetone, isopropyl alcohol, and
deionized (DI) water, the negative-tone electron beam resist HSQ (formulated as product no. XR-1541-006, Dow Corning) was spin-coated
onto quartz substrates with a thickness of 100 nm, followed by spincoating 10 to 20 nm of water-soluble conducting polymer (Espacer) as
the charge-dissipating agent during EBL process. To avoid thermally
induced cross-linking of the resist, which would weaken the developing contrast and lead to the worse resolution, no prebaking process
was used. The designed structure was created by AutoCAD in dxf format. Then, the HSQ patterns were lithographically defined by EBL
using an Elionix ELS-7000 system with an accelerating voltage of
100 kV and dose of 5200 mC/cm2. The pattern was arranged in 100 ×
100 mm, and no proximity effect correction was performed for the exposure. After patterning, we washed out the Espacer by DI water first,
and then a high-contrast develop process was used by developing samples in a formulation of aqueous 1% NaOH and 4% NaCl in DI water
at room temperature for 1 min, followed by rinsing with DI water for
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particular, if Ea is the electric field on an aperture in a perfectly
conducting screen, the aperture is equivalent to a transverse magnetic
current Jm ¼ −n Ea on the aperture plane (47). Although this equivalence is exactly valid only for perfectly conducting screens, it also approximately holds for metallic sheets at optical frequencies, provided
that the screen is sufficiently opaque, for example, the thickness of the
metallic screen is larger than the skin depth in the wavelength range of
interest (48). To show the complementary response of the two metasurfaces, we numerically investigated the transverse electric current Js
and the effective transverse magnetic current Jm induced on the subunit cell under antisymmetric excitation. In particular, Fig. 5 (B and C)
shows the Js distribution on the nanoantenna at the top layer and the
Jm distribution on its complementary Babinet-inverted structure at the
bottom. The transverse electric current Js flows, with high intensity along
the V-shaped nanoantenna (Fig. 5B), whereas the transverse magnetic
current Jm is very weak in the same region (not plotted), as expected.
Meanwhile, at the bottom complementary structure, the effective transverse magnetic current Jm is concentrated in the aperture region and
flows from the two tips toward the apex of the V shape, as shown in
Fig. 5C. The induced currents for symmetric excitation also exhibit similar behavior (see fig. S6). Because the incident plane wave impinges
from the side of the nanoapertures, the problem of electromagnetic
transmission through the bilayer metasurface can be equivalently studied
as a radiation problem from the magnetic currents induced on the apertures. The radiation from these current elements is affected by the presence of the nanoantennas. If their distance is properly tailored, a high
radiation efficiency can be realized, which in turn corresponds to a high
transmission efficiency. These considerations provide a different perspective on how the radiation symmetries are broken by the presence
of the two metasurfaces.
From a ray optics perspective, there is no direct light path from
the input surface to the output surface because of the complement
of two layers. Moreover, according to Bethe’s predictions (49), for a
subwavelength aperture in an ultrathin metallic sheet, transmission
is inversely proportional to the wavelength’s fourth power, which
would imply a very small transmission. However, simulation results
showed that the total transmission of our bilayer metasurface (copolarized and cross-polarized) has a peak of 38%, around 780 nm. This
value is even larger than the ratio of the slot area to the total area of
the gold film, which is about 30.3%. This large transmittance may result from the phenomenon of extraordinary transmission of light
through small apertures, which involves the excitation of leaky plasmon modes on periodic metallic screens, and has been extensively
studied in recent years (50–53).
It should be noted that our design mechanism is different from
previous metasurfaces, which relies on the creation of a library correlating phase and amplitude information of the scattered light with the
corresponding geometrical parameters of one subunit cell. Then, a
supercell could be formed to cover the required phase range in a discretized fashion. The prerequisite of such a procedure is based on the
assumption that the mutual coupling among adjacent subunits is negligible. On the contrary, our design principle exploits not only the intralayer coupling but also the interlayer coupling to enhance the overall
transmission and the conversion efficiency. Therefore, to design the
geometry at a given wavelength, we need to tailor the parameters of
all subunit cells as a whole by considering the coupling of all neighboring elements. Our findings show that the generalized Snell’s law is
still operational in the presence of strong mutual coupling. The working
RESEARCH ARTICLE
2 min, and drying the sample by N2 blowing. After EBL patterning, a
layer of 30-nm gold was deposited on the sample by electron beam
evaporator (Explorer Coating System, Denton Vacuum). Lift-off process is not needed. The fabricated structures were imaged using Elionus
ESM-900 scanning electron microscope with an accelerating voltage of
10 kV. The surface profile of the sample was measured by an AFM
(ICON-PKG, Bruker), as shown in fig. S1.
The bilayer metasurface is constructed by lifting the V-shaped gold
nanoantennas above their Babinet-complementary structures with
conformal HSQ pillars. The shape and size of the nanoantennas on
the top layer are the same as the Babinet-inverted apertures on the bottom gold film. The distance between the two layers is only 70 nm, implying that, from a ray-optic perspective, there is no direct light path from
the input to the output windows. Additionally, the width of the slots is
55 nm, and their length is around 130 nm, which are much smaller
than the working wavelength in the visible range.
∫real
Ex conjðHy Þ dS
Source power
ð1Þ
∫real
−Ey conjðHx Þ dS
Source power
ð2Þ
1
TEx ¼ 2
TEy ¼
1
2
Optical characterization
Experimental characterization of the fabricated structures was performed by a home-built microregion spectrum testing system. A
schematic of the setup is shown in fig. S9. A low-power supercontinuum laser (SC450-6, Fianium), tunable from 300 to 2000 nm,
was weakly focused on the sample using a plano-convex lens with
focal length F = 60 mm. A polarized beam splitter prism was used
before the focusing lens to obtain Ey–linearly polarized incidence
light, and a polarizer was used after the sample to detect the polarization property of the anomalous and normal transmission beams.
Scanning the wavelength of a supercontinuum laser, the transmitted
Qin et al. Sci. Adv. 2016;2:e1501168
1 January 2016
SUPPLEMENTARY MATERIALS
Supplementary material for this article is available at http://advances.sciencemag.org/cgi/
content/full/2/1/e1501168/DC1
Deviation between experiment and simulation
Effect of higher-order diffraction mode
Dependence of resonant wavelength on geometric parameters
Fig. S1. Surface profile measured by AFM.
Fig. S2. Tilted SEM image of the fabricated bilayer metasurface, showing nonuniform grains of
gold at the sidewall of HSQ pillars.
Fig. S3. Dependence of the extinction ratio on the thickness of cladding layer and the
thickness of deposited gold film.
Fig. S4. Refraction efficiency of the cross-polarized component in each diffraction orders.
Fig. S5. Electric field distribution on the top layer of the bilayer metasurface at 770 nm.
Fig. S6. Transverse currents distribution under symmetric excitation.
Fig. S7. Dependence of the conversion efficiency on the width of nanoantennas.
Fig. S8. Dependence of the conversion efficiency on the length of nanoantennas.
Fig. S9. Schematic representation of the optical measurement setup.
Fig. S10. Dependence of the conversion efficiency on the height of the HSQ pillars.
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Acknowledgments: We thank K. Huang for the helpful discussion and suggestion. Funding: This
work was supported by the National Research Foundation, Prime Minister’s Office, Singapore
under its Competitive Research Program (CRP award no. NRF-CRP10-2012-04); F.M. and A.A. were
supported by the Air Force Office of Scientific Research (grant no. FA9550-13-1-0204) and The
Welch Foundation (grant no. F-1802). Y.L. acknowledges the support from Major Program of
the National Natural Science Foundation of China (grant no. 61490713); and S.Z. acknowledges
the financial support by The Leverhulme Trust (grant no. RPG-2012-674). Author contributions:
F.Q. and C.-W.Q. conceived the idea. F.Q., L.Z., and F.M. conducted the numerical simulations and
contributed to the physical interpretations. L.D., C.C.C., and J.D. fabricated the samples. F.Q., L.D.,
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Data and materials availability: All data needed to evaluate the conclusions in the paper are
present in the paper and/or the Supplementary Materials. Additional data related to this paper
may be requested from the authors. All data, analysis details, and material recipes presented in this
work are available upon request to C.-W.Q.
Submitted 26 August 2015
Accepted 25 October 2015
Published 1 January 2016
10.1126/sciadv.1501168
Citation: F. Qin, L. Ding, L. Zhang, F. Monticone, C. C. Chum, J. Deng, S. Mei, Y. Li, J. Teng,
M. Hong, S. Zhang, A. Alù, C.-W. Qiu, Hybrid bilayer plasmonic metasurface efficiently
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Hybrid bilayer plasmonic metasurface efficiently manipulates visible light
Fei Qin, Lu Ding, Lei Zhang, Francesco Monticone, Chan Choy Chum, Jie Deng, Shengtao Mei, Ying Li, Jinghua Teng,
Minghui Hong, Shuang Zhang, Andrea Alù and Cheng-Wei Qiu
Sci Adv 2 (1), e1501168.
DOI: 10.1126/sciadv.1501168
http://advances.sciencemag.org/content/2/1/e1501168
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